The reliability of aluminum-based interconnections in microelectronic circuits is generally limited by a phenomenon known as electromigration. As microelectronic circuits are made more dense in order to improve performance, the electric fields (and resulting current densities) in the aluminum interconnects increase. Hence, as circuit densities increase, the rate of electromigration also increases.
Electromigration leads to circuit failure primarily via two mechanisms. In the first, aluminum electromigrates away from a region in the interconnect faster than the availability of additional atoms can take its place. This process generates a void in the interconnection. Growth of this void increases the electrical resistance of the interconnection to a point where circuit failure occurs. The second means by which electromigration failure occurs is when metal electromigrates into a region faster than it escapes the region, thus locally piling up metal atoms (called extrusions) to a point where it extends to the adjacent interconnection line, thereby causing an electrical short circuit.
The problem of electromigration has been approached in a number of ways, the two most common are: (1) introducing a second species into the parent metal, e.g., alloying aluminum with a 0.2% to 4% copper, and (2) utilizing a redundant metal layer, e.g., titanium, tungsten or chromium layer(s) under and/or over the aluminum lines.
The addition of Cu into the Al line has been described by Ames, et al., in U. S. Pat. No. 3,879,840, and of common assignee. The formation of a thin intermetallic layer within the Al-Cu has been described by J. K. Howard in U.S. Pat. Nos. 4,017,890 and 4,154,874, and of common assignee. While the above methods increase electromigration lifetime, it has been found that electromigration failure cannot be totally avoided because void growth cannot completely be suppressed, i.e., void sizes increase with time. The use of redundant layers can extend the electromigration lifetime of metal lines by a factor of 10 to 100 over non-redundant schemes. The present invention, however, offers a lifetime extension in excess of 1000.times.. This is because with the present invention, void growth is totally eliminated, which is made possible by a phenomenon known as the short-length effect.
The short-length effect takes place in short aluminum interconnections if an electrical current is supplied through leads of materials in which aluminum diffusivity is low. The physical origin of the short-length effect is the build-up of backstress as aluminum atoms pile up against the diffusion barrier leads; this backstress counteracts the electromigration driving force. A steady-state condition arises in situations where the backstress exactly balances the electromigration driving force. Under this condition, no further electromigration damage results.
The existence of the short-length effect has been demonstrated by several investigators as, for instance, by H. V. Schreiber in the article: "Electromigration Threshold of Aluminum Films" published in Solid State Electronics, Vol. 28, No. 6, p. 617, by R. G. Filippi, et al., in the article: "Evidence of the Electromigration Short-Length Effect in Aluminum based Metallurgy with Tungsten Diffusion Barriers" published in the Proceedings of the Materials Research Symposium, Vol. 309, pp. 141-148, and by X. X. Li et al., in the article: "Increase in Electromigration Resistance by Enhancing Backflow Effect" published in the Proceedings of the 30th International Reliability Physics Symposium, March 1992, p. 211. Based on experimental evidence, an interconnection of length L will show electromigration immunity for an applied current density below a critical value j.sub.cr. This j.sub.cr has been found to be inversely proportional to the interconnection length L. The above investigators of the short-length effect use various schemes to prove the existence of this phenomenon. However, these schemes do not utilize the self-aligned features and fine pitch which are essential for the manufacturability of Very Large and for Ultra Large Scale Integration circuits (VLSI and ULSI).